(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a cost-effective method of creating a substrate that can be applied for flip-chip packages.
(2) Description of the Prior Art
High density interconnect technology frequently leads to the fabrication of multilayer structures in order to connect closely spaced integrated circuits with each other. A single substrate serves as an interconnect medium to which multiple chips are connected, forming a device package with high packaging density and dense chip wiring. The metal layers that make up the interconnect network and the via and contact points that establish connections between the interconnect networks are separated by layers of dielectric (such as polyimide) or insulating layers. In the design of the metal interconnects, strict rules must be adhered to in order to avoid problems of package performance and reliability. For instance, the propagation directions of primary signal lines must intersect under angles of 90 degrees to avoid electrical interference between adjacent lines. It is further required that, for considerations of photolithography and package reliability, planarity is maintained throughout the construction of multi-layer chip packages. Many of the patterned layers within an interconnect network form the base for subsequent layers, their lack of planarity can therefore have a multiplying effect of poor planarity on overlying layers.
Quad Flat Packages (QFP) have in the past been used to create surface mounted, high pin count integrated packages with various pin configurations. The electrical connections with these packages are typically established by closely spaced leads that are distributed along the four edges of the flat package. This limits the usefulness of the QFP since a high input/output (I/O) count cannot be accommodated in this manner. To address this problem, the Ball Grid Array (BGA) package has been created whereby the I/O points for the package are distributed not only around the periphery of the package but over the complete bottom of the package. The BGA package can therefore support more I/O points, making this a more desirable package for high circuit density with high I/O count. The BGA contact points are solder balls that in addition facilitate the process of flow soldering of the package onto a printed circuit board. The solder balls can be mounted in an array configuration and can use 40, 50 and 60 mil spacings in a regular or staggered pattern.
Increased input/output (I/O) requirements combined with increased performance requirements for high performance Integrated Circuits (IC""s) has led to the development of Flip Chip packages. Flip chip technology fabricates bumps (typically Pb/Sn solder) on Al pads and interconnects the bumps directly to the package media, which are usually ceramic or plastic based. The flip-chip is bonded face down to the package through the shortest paths. These technologies can be applied not only to single-chip packaging, but also to higher or integrated levels of packaging, in which the packages are larger, and to more sophisticated package media that accommodate several chips to form larger functional units. The flip-chip technique, using an area array, has the advantage of achieving the highest density of interconnection to the device combined with a very low inductance interconnection to the package.
It is the objective of packaging ball grid array and flip-chip packages that the chip is mounted on the surface of a package substrate. The contact points of the flip-chip Integrated Circuit (IC) device make contact with contact points in the top surface of the Ball Grid Array (BGA) substrate, the substrate re-distributes (fan-out) the flip-chip IC contact points. The lower surface of the substrate has the contact point (balls) that are connected to the surrounding circuitry and that form the interface between the BGA/flip-chip IC contact balls and this surrounding circuitry. It must thereby also be understood that the original contact balls of the flip chip IC device are encased in a material (for instance epoxy) for protection of these balls. The epoxy is encased between the lower surface of the flip-chip IC device and the upper surface of the BGA substrate. This epoxy layer is referred to as underfill since it is filled in under the flip-chip device. The underfill is normally put in place using a separate process of dispensing epoxy liquid under the die followed by curing of the epoxy. IC devices that are packaged using a flip chip and that have requirements of high power dissipation normally require a heatsink that is attached to the surface of the flip chip die. Only the backside of the flip chip is exposed and is suitable for the attachment of the heatsink. Since the heatsink is only attached to the (backside) surface of the IC device, great care must be taken not to induce stress on the backside of the IC device. If too much force or stress is used during the process of attaching the heatsink to the die, the die can easily crack or break. If on the other hand a larger surface area is created that is parallel to the surface of the backside of the IC device, the stress can be significantly reduced.
Existing solder-mask coating for a flip-chip substrate and the formation of a pattern of openings thereto uses either techniques of Photo-imageable Solder Resist (PSR) or thermally cured ink with laser ablation for both solder bump and contact ball pads. The creation of solder bump pad openings and the creation of PSR openings require significantly different levels of accuracy, with a process requiring a higher level of accuracy typically being characterized by low productivity and high cost. The invention provides a cost-effective and reliable method that addresses these concerns.
A principle objective of the invention is to provide a method of creating a substrate for application in a flip-chip package that is cost effective and reliable.
Another objective of the invention is to apply the creation of high-accuracy and low-accuracy openings overlying points of electrical access over the surface of a semiconductor device supporting substrate in a cost-effective and reliable manner.
In accordance with the objectives of the invention a new method is provided for the creation of high-accuracy and low-accuracy openings overlying points of electrical access over the surface of a semiconductor device supporting substrate. Openings are first created for access to the substrate followed by copper plating and then patterning of the plated layer of copper, creating the interconnect metal over the surface of the substrate. A first solder mask is coated over the surface of the substrate, this first solder mask must be provided with a first array of low-accuracy openings for electrical access there-through for the placement of contact balls. The first openings can be created using conventional film artwork since low accuracy is required for the contact ball openings, resulting in a low-cost process for the creation of the first openings. A second solder mask is next coated over the surface of the first solder mask. Through this second solder mask is created a second array of high-accuracy second openings that provide access to solder bumps using methods of laser ablation, resulting in a low-throughput process which however is only applied to create access to solder bumps.